I know someone brought this up, but asking for it to solve a problem that he already found a work around for.
It would be a great addition though, have the ability to assign a Press & Hold (Long Press) Key Binding on a button. Maybe also have the Long Press Delay (when you determine it's really a long press) be configurable via the UI, so that people that tweak it so that the Single Click and Long Press are responsive to their play style.
Press A = Spacebar
Press A for 2 Seconds = X
If there is no long press binding set, then it will immediately call the normal Press event, in this case Spacebar. Then the 2 second delay could be set either per button, as a global value, Or Both (so it will use the global value by default, but it can be overriden per key).